A known problem with memory elements that include a solid ion conductor layer, such as conductive bridging random access memory CBRAM type elements (also called programmable metallization cells (PMCs)) can be variability in resistance response between elements and between cycles of a given device.
A conventional programming operation is shown in FIGS. 20A to 20E. FIG. 20A is a timing diagram showing a conventional programming pulse voltage (Vprog(Conv)) that can include stepped increases in potential over time.
FIGS. 20B to 20D are diagrammatic representations of a CBRAM element 2091 response to one or more conventional programming pulses. A conductive filament 2099 can be formed between two electrodes (2097, 2095). FIGS. 20B to 20D show how application of a potential, like that shown in FIG. 20A, can eventually create a conductive filament 2099 through an ion conducting layer 2093.
While a conventional programming operation can form an initial conductive path (shown in FIG. 20D), as shown in FIG. 20E, a filament configuration can change 2099′ (i.e., narrowing or opening portions of a conductive filament), resulting in an increase in filament resistance. It is believed that such a change in filament structure can be spontaneous, or can occur in response to other electrical fields applied to the element (e.g., read disturbs, etc.).
In work unrelated to CBRAM type memory elements, physical properties of atomic sized metallic conductors/wires are summarized in Physics Reports Volume 377, Issue 81 (2003). The article shows one method of making an atomic sized metallic wires (AWs) that includes driving an STM (scanning tunneling microscope) tip into a metal sample surface, then retreating the tip, to yield a chain of atoms between tip and the sample. The article comments that by moving the tip back and forth (up and down) by small amount about the point at which the wire connects/disconnects, the conductance of the connected configuration can be made more repeatable. Evidence suggests that the atoms of such a wire (which can be Au) can have certain preferred configurations that are more stable (i.e., do not change over time) than other configurations.
The article also describes a second characteristic of atomic wires made by the STM method. When an STM tip is brought extremely close to surface, a sudden “jump to contact” is observed. This jump occurs with a complete metallic contact being suddenly formed between the tip and surface. The jump to contact of AWs is associated with the bonding force between the tip and substrate. As the tip gets very close (2 or fewer angstroms), the bonding force pulls so strongly on the tip and substrate that the atoms away from the contact point are strained, lengthening the tip a bit. Bringing the STM tip close, then far, then close etc., to the surface may allow the atoms at the tip to wiggle into a preferred configuration.